Oil tank breather

The blanked off pipe you refer to is an open pipe on my Combat that has the oil tank vent connected to it. 

The new E start from Norvil when fitted requires a shorter version air filter unit that doesn't have provision for the oil tank vent. 

Obviously the oil tank needs to be vented so, If it was left up to me I'd fix it over the rear chain so as to avoid lubricating the rear tyre, or better still unblank the air filter tube and fit the dangling pipe to it.

The oil tank breather was piped to the back of the air filter on your, and earlier, bikes. But people would remove it and put the outlet somewhere else because 1) the oil filter end up soaked in oil and 2) oil dripped down on the gearbox.

Directing the outlet at the chain was a popular choice but did lead to a very oily chains spraying oil over the back tyre.

Fixing the engine breathing is probably the place to start but there can still be a problem with oil from the sump being ejected when you start it after standing for a while. And some sort of oil seperator in the breather line to knock out excess oil and return it to the tank, rather than using the oil tank itself for this purpose, feeding the much cleaner blow-by gas into the airbox.

For now, I'd stick the end of the breather pipe into a catch tank, such as a 500ml beer can cable-tied the rear frame loop, and see how much oil you have to deal with.

The one in the diagram is from a Mk2A / MK3, which Andover will sell you for about £22.

I'm not happy that the 'condensed' oil ends up in the inlet manifold; I'd rather it went back in the tank. But it is something you could readily fit to your bike and, together with the breather valve modifications you'll find on this forum, should allow you to route breather gasses into the air box and not fill it with oil.